FRSA Staff
When you mention “blue roofs” in Florida, there is the tendency to think damaged roofs that have been tarped, often followed by the thought of how the was tarp installed. Rarely would we think of an actual roofing system that is being used in the U.S. as well as in other areas of the world.
A blue roof is a roof that is designed for rainwater retention above the waterproofing membrane. This is different from more standard roof designs that allow for swift water drainage to avoid deterioration or other wear occurring over time.
A blue roof is typically flat without any falls, and control devices regulate drainage outlets from the roof, enabling water to be stored temporarily or permanently. They can be designed as open water surfaces, as storage within or beneath a porous or modular surface, or below a raised surface or covering.
Some reasons for incorporating a blue roof into a building include:
■ Sustainable Urban Drainage System (SuDS) used to mitigate urban flooding caused by stormwater run-off.
■ Reduction of the amount of rainwater attenuation required, in areas of urban density, for the green roof that is typically installed over the blue roof.
■ Rain water harvesting, which may be a planning or building regulations requirement.
■ Storage for re-use purposes such as green roof irrigation, cooling of solar panels, or reduction of HVAC load on mechanical refrigeration equipment.
■ Recreational purposes, such as rooftop swimming pools or water features.
Unlike other SuDS, blue roofs are able to make use of otherwise redundant building space – flat roofs – without extending beyond the footprint of the building or into ground space which, depending on the density of the location, may be expensive. Because they restrict the maximum permissible discharge of stormwater during periods of heavy rainfall, SuDS help reduce the risk of sewer overflow and localized flooding.
In 2000, the roofing industry got behind the emerging demand for green roofing, which was being led significantly by the London Borough Planning Offices, the Mayor’s "London Plan" and organizations such as livingroofs.org.
While there are many positive drivers behind the reasons for using green roofs, the main drivers could be seen as incident rainfall control, air pollution reduction and biodiversity. Fast forward through the evolution of green roofs and so-called "brown roofs" (un-planted roofs left to nature) and we find that the roofing industry is now investing in the development of blue roofs.
Consisting of a water storage medium, typically a cellular geocomposite drainage/retention board, and a discharge control mechanism, blue roofs reduce rainwater discharge rather than drain it as quickly as possible. Correctly designing the blue roof releases attenuated water at the required discharge rate for a site, providing the integral source control and attenuation required by a SuDS system. All this without the requirement for land consuming ponds, systems, can range from simple to highly sophisticated.
Some actively controlled systems will use programmable, hydraulically-controlled valves to control the retention and release of water for reusable collection, drainage, or subsurface storage in a series of containers. Some even use communications or data tools such as forecast integration, a technology employing sensors and internet-based data feeds to help estimate rainfall quantities. Projects can use such tools to help schedule and record their real-time rainwater collection to meet user demand.
Whereas passive blue roofs typically need little to no upkeep, one of the primary considerations of active blue roofs is their need for regular maintenance. Passive blue roofs simply catch and hold water, and may also function as temporary holding tanks for storm events by later releasing water via evaporation.
One true beauty of blue roofs is they can be relatively low in cost when they are carefully and strategically designed. With an installation price often close to or less than one dollar per square foot, blue roofs provide a relatively inexpensive yet effective tool to help manage rainwater collection. By comparison, a vegetated roof can cost from $15 to $20 per square foot. Some projects, using a combination of both blue and green roofs, are able to achieve the financial balance necessary to meet a given budget.
By any account, existing structure is a primary consideration. As noted, the overall volume of a blue roof system depends on its geometry and configuration. When being installed on existing roofs, the new materials and water loads must be carefully planned to not exceed the existing roof structure’s load capacity. When the projected loads are designed to be greater than the current capacity of the existing roof structure, structural engineers must carefully calculate and add the additional structure required (e.g. joists, beams, or other structural members) to help carry projected water loads.
In addition to stormwater management and rainwater collection, blue roofs can also provide highly beneficial secondary outcomes. They can offer substantial heat island cooling effects by providing the opportunity to use basins with white or reflective linings that deflect solar radiation from the building.
The lifespan of a typical blue roof is generally estimated at around 35 years. This is based on concepts of normal wear and tear. After 30 or 40 years of exposure to the sun and the elements, depending on the details and specifications for the project, synthetic materials (especially plastics) can begin to disintegrate, valves and rubber gaskets can harden, and some metal fasteners or flashings can rust.
Nevertheless, it is evident that a reasonable upfront investment in a blue roof system can easily yield many years of productive stormwater collection and rainwater diversion. Along with savings in water usage and energy consumption, blue roofs have proven to be highly beneficial for a building owner over the long term.
In some ways, green roofs and blue roofs accomplish similar things. Green roofs consist of growing plants on rooftops and can be divided into two groups: extensive and intensive. Extensive green roofs refers to systems that usually have six inches or less of growing medium and often are placed on top of the roof in either trays or bags. Intensive roofs are deeper installations that include full continuous soil over an integrated drainage mat and water proofing. These roofs can range from eight inches to several feet deep.
In contrast, the term “blue roof” has been coined to refer to systems that focus on rainwater collection. By using catchment pools, rain barrels and more discreet water-hungry plants, the goal of the roof system is to minimize the amount of storm water that a building site sheds to the rest of the city.
Both blue and green systems help mitigate stormwater by gathering and releasing it over time. Both offer an insulating layer to help trap energy in the winter and reflect sunlight in the summer (reducing the heat island effect in urban centers.) The difference is that green roofs offer an opportunity for biodiversity and food production while the water gathered by blue roofs can be used for irrigation, cleaning sidewalks or reducing potable water use by filling washing machines or toilets. En masse, both systems will be valuable, productive additions to the top of New York's skyline and many other U.S. cities.
Information for this article was gathered from multiple sources including: One Roof, Two Roofs, Green Roofs, Blue Roofs; Blue Roof and Green Roof; Green & Blue: Joined Up Thinking to Reduce Flood Risk; LivingRoofs.org; The Rise of the Blue Roof; From Green to Blue: Making Roofing Systems Sustainable in Urban Environments.
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